With the advances in technology related to integrated circuits (ICs) and in particular to surface mount IC's and more particular to ball grid arrays (BGAs) and chip scale packages (CSPS) and flip chips (FCs) in addition to the density of ICs utilized in electronics devices it has become increasingly difficult to visually inspect the integrity of the soldered connection between the chip leads and the solder pads on a printed circuit board. In addition, the number of soldered connections per chip has increased while inversely the size of the chip has decreased. While some surface mounted ICs have soldered connections which can be visually inspected because the chip leads project from the edge of the IC, the above-referenced chips cannot be visually inspected without an inspection device because the chip leads project perpendicular to the surface of the IC, and therefore the soldered connections are hidden by the IC.
One method to test the integrity of a soldered connection is that shown in U.S. Pat. No. 6,288,346 Hirofumi et al. wherein a plurality of test lands are disposed upon the circuit board which are connected to solder pads to which a BGA package is to be soldered to in which the continuity between the pins on the BGA can be confirmed. While this inspection method may disclose an open connection it cannot distinguish if there is a ‘bridged-connection’ that is where a ball of solder connects more than one pin. A bridged connection may lead to component failure when power is applied to the IC, therefore it is desirable to determine if bridged connections exist. Additionally, the testing method of Hirofumi et al. does not disclose the quality of the soldered connections, that is whether the solder was not heated to a high enough temperature. Therefore, the expected lifetime of the soldered connection cannot be estimated which may lead to more product failure after sales.
Another method to check soldered connections for the above-referenced IC's is through the use of x-rays. With x-ray inspection, it can be determined whether there are open connections, bridged connections and if the BGA was properly aligned with the solder pads of the printed circuit board. It is not possible to determine the quality of the soldered joint, as described above. In addition, it cannot be determined if excess flux residue remains within the soldered connections. Still further, the use of x-ray inspection requires dedicated equipment in addition to requiring protection from radiation exposure from the x-ray testing device. Lastly, x-ray inspection units require a skilled operator to utilize the device, thereby leading to complexity as well as costs to the overall product.
Still yet another method of inspection that is known is the production of a micrograph in cross-section through a soldered connection. This requires destructive testing, wherein a circuit board is taken from the assembly line and a cut is made passing through the IC to visually inspect the soldered connection. Though, this method will only produce an estimate of the actual soldered connections and requires that conclusions must be drawn as to the operating parameters of the soldering process.
Another known process of inspecting soldered connections is through the use of devices which can transmit images from one location to another, such devices include endoscopes and borescopes. These devices generally have a cylindrical profile and include a plurality of lenses disposed therein for the transmission of an image therethrough. A shortcoming of these devices is that at one end of the device there is disposed a light source adjacent to an image collection device. The light source is utilized to illuminate the area adjacent to the image collection device wherein an image is then reflected into the endoscope and transmitted to the opposite end. The clarity of the transmitted image may be diluted due to excess light emitted and/or reflected from the light source which is transmitted through the device.
Referring now to U.S. Patent Application Publication No. 2001/0024273 Cannon, there is disclosed yet another device for the inspection of soldered connections. In particular, the inspection device shown and described in the above-referenced patent application can be utilized to visually inspect soldered connections of BGA, CSPS, and FCs. The device includes an ocular unit, a lens head, and image transmission unit for transmitting the image receive by the lens head to the ocular unit and an illuminating device. As shown and described in Cannon the device therein may be utilized by placing the lens head adjacent to a BGA to be inspected. An illumination source illuminates the soldered connections while a second illumination source is utilized to backlight the soldered connections. A prism assembly disposed within the lens head receives a reflected image of the soldered connections, the reflected image is transmitted through an image transmission unit and into a camera. An aperture is disposed between the transmission unit and the camera to control the image received by the camera. A shortcoming of the device of Cannon is that the image reflected through the image transmission unit contains “interference” which leads to the degradation of the image. The term “interference” refers to the excess reflected light which will be transmitted through the image transmission unit. This excess light will combine with the image to be view, wherein the final combination of the image to be viewed and the interference will then be filtered by the aperture disposed adjacent the camera. An additional shortcoming of the device of Cannon is that the field of view of the lens head is to narrow to visually inspect both the upper solder connections as well as the lower solder connections, in order to visually inspect both, the lens head must be moved away from the soldered connections to provided a greater field of view, though at the cost of clarity of the image. A further shortcoming of the Cannon device is that a prism is utilized to reflect the image of the soldered connections, it is well known that prisms tend to be brittle and therefore require protection. For example, as shown in Cannon the prism is protected by webs, these webs extend beyond the edge of the prism, therefore the leading edge of the prism cannot be lowered such that the prism contacts the circuit board because of the protection webs. Lastly, prisms are very expensive therefore increasing the overall cost of the inspection device, as well as requiring specially trained technicians for repairs and/or servicing of the prism assembly.
Therefore there is a need for a device and methods of use which will enable the visual inspection of soldered connections, wherein the device provides a clear image of both the upper and lower connection without having to readjust the focal length of the device. Additionally, there is a need for an optical inspection device that eliminates interference within the transmitted image, thereby providing a better image of the soldered connection.